1. Field of the Invention
The present invention relates to an optical attenuator, and more particularly, to an optical attenuator using a multi-mode interference.
2. Description of Related Art
An optical attenuator is one of very important elements of an optical circuit for an optical communication to control an optical signal transmission. The optical attenuator performs an add/drop function to equalize an optical power of channels of a wavelength division multiplexing (WDM) system. Also, when a total power of an erbium doped fiber amplifier (EDFA) is varied, the optical attenuator flats a gain in order to prevent an output variation according to a wavelength. The optical attenuator also controls a magnitude of a receiving signal according to a maximum signal magnitude of a receiving end so that the receiving end may not be overloaded.
As the optical attenuator, a fiber optical attenuator is wide in use. However, the fiber optical attenuator is large in size and is complicated in manufacturing process. Therefore, manufacturing yield is low, and a production cost is high.
Also, a conventional optical attenuator based on a planar lightwave circuit (PLC) technique (e.g., disclosed in Korean Patent Publication No. 1999-20073) uses a directional coupler. Thus, the optical attenuator is large in length and wide in width, and is narrow in manufacturing process margin. In addition, since a bent waveguide has to be used at various locations due to the directional coupler, a length of the optical attenuator becomes larger, and a loss also becomes larger.
To overcome the problems described above, preferred embodiments of the present invention provide an optical attenuator having a wide manufacturing process margin.
It is another object of the present invention to provide a small-sized optical attenuator.
It is another object of the present invention to provide an optical attenuator having low power consumption.
In order to achieve the above object, the preferred embodiments of the present invention provide an optical attenuator, comprising: a first multi-mode waveguide varying a phase of an optical signal input through an input terminal;
first and second single-mode waveguides having an input terminal coupled to an output terminal of the first multi-mode waveguide and varying a phase of the optical signal by a thermo-optic effect by electrodes attached thereto; and a second multi-mode waveguide having an input terminal coupled to an output terminal of the first and second single-mode waveguides and varying a phase of the optical signal, wherein a predetermined optical power distribution obtained by offsetting or adding up a phase varied in the first and second multi-mode waveguides and a phase varied by the thermo-optic effect is transferred to an output terminal of the second multi-mode waveguide.
For the sake of an additional attenuation, two or more optical attenuators can be serially connected to each other.
A length of the first and second multi-mode waveguides depends on a wavelength of the optical signal and a material thereof. The input terminals and the output terminals of the first and second multi-mode waveguides are located at a bisectional point or a trisectional point of a width of the first and second multi-mode waveguides. A phase varied by the thermo-optic effect is in a range between 0xc2x0 to 180xc2x0. A phase varied in the first and second multi-mode waveguides is 0xc2x0 or 270xc2x0.
The optical attenuator according to the present invention has the following advantages: since the optical attenuator employs a multi-mode waveguide and thus can exclude a bent single-mode waveguide, a manufacturing process margin is large. Also, a length of the optical attenuator to obtain a certain optical power distribution becomes short. In addition, power consumption can be reduced by lowering an applied voltage of the temperature control electrodes.